Solid sealant with environmentally preferable corrosion resistance

ABSTRACT

The faying surface of one or of both of a pair of substrates is provided with a layer of solid polyurethane sealant which is adherent to at least one of the respective surfaces. The substrates are additionally fastened together so that the faying surfaces compress the polyurethane sealant between them to form a fluid seal across the entire spacing between the two surfaces, and between the polyurethane sealant and the faying surfaces themselves.

FIELD OF THE INVENTION

Sealing against passage of fluids—gases or liquids—between two fayingsurfaces which confront one another with a layer or layers of solidsealant in contiguous and continuous fluid-sealing contact with thesurfaces, which include an environmentally preferable corrosionresistance component.

BACKGROUND OF THE INVENTION

Sealing against the passage of fluids—gases or liquids—between fayingsurfaces is a very old art. “Faying” surfaces are those which directlyabut each other, or which almost abut each other with an intermediatebody between them such as a sealant. Built-up structures are repletewith faying surfaces. The substrates bearing the surfaces arecustomarily joined together by fasteners such as rivets or threadedcombinations whose purpose is to hold them against separation and tolimit or prevent relative shear movement between them.

Especially in structures which are subject to a variety of environmentssuch as substantial temperature change, and deflection due to bending orvibration, relative movement and separation between local regions ofconfronting faying surfaces cannot be completely prevented. Thus eventhose flat surfaces which are practically attainable when broughttogether will in use permit leakage of fluid between them.

This is a tolerable situation where the confinement of a fluid is notrequired. Many faying surfaces are joined without regard for sealing.However in applications such as fuel tanks, leakage of fuel is nottolerable. In aircraft fuselages, leakage of air from a pressurizedcabin must at least be minimized. There are numerous other examples inaircraft and spacecraft as well as in ground based structures wherefluid containment (gas or liquid) in a built-up structure is required.

This is an old problem, and it has been solved in various ways whichutilize sealants applied to appropriate parts of the structure. Withrivets or fasteners in areas where sealing is critical, beads of sealantare applied around exposed edges of adjacent surfaces, often in the formof wet sealants which cure or dry after assembly. The techniques forapplication and subsequent clean-up are both expensive and laborintensive, and complicate the assembly, maintenance and repair of thestructure.

In aircraft construction, a wet layer of sealant is often placed betweenthe faying surfaces at the time they are joined together, and thesealant cures after the assembly is completed. The process is designedin such a way that the entire interstitial area will be filled. Toassure this, an excess of sealant is applied before the surfaces arebrought together. Some of the sealant is expelled from between thesurfaces when they are joined, and their excess sealant must be removedfrom the adjacent area around the sealed edges. The problems created bythe excess sealant are not trifling. The surrounding area becomes a messthat has to be cleaned up, and all excess sealant must be carefullyremoved. If the sealant contains toxic additives such as chromates, thetoxic excess requires careful disposal methods which are expensive. Theexpensive disposal extends to auxiliary items such as the cloth used forremoval, brushes used for application, and the like. Worker protectionmust be provided against contact with such a sealant, requiring the useof masks and gloves. Solvents such as 1,1,1-trichloroethane and ethersolvents which are used for the cleanup bring their own hazards.

In order to assure adequate filling of the interstitial region, it isnot sufficient merely to provide an excess of sealant. It is alsonecessary to provide a uniform excess. This requires a further toolingstep to rake and trim the exposed wet surface to a uniformly thick areaof a configured shape.

These labor intensive procedures are costly. They must be performed areasonably short time before assembly so the sealant remains fluid whilethe assembly is completed. This is a serious limitation on the freedomto schedule production, because the surfaces cannot be prepared longbefore assembly and then wait their turn for use.

The above labor and economic problems and shortcomings of a sealantwhich is applied wet at the time of assembly are severe. They alsoinvolve the economic problems of material waste and structural weightpenalties. The waste of expensive sealant material which must be wipedup and disposed of is the lesser of these.

Of far greater consequence is the weight penalty. It has been calculatedthat on a large aircraft such as the C-17, the use of the dry sealantsystem of this invention can reduce the total sealant weight required byabout 800 pounds compared to the weight of a wet sealant even when thewet sealant is applied in an optimum manner. It should be rememberedthat weight is an extremely expensive quantity in aircraft andspacecraft, because each pound and structure to support it requires fuelto raise it every time it is lifted. It has been estimated that inaircraft, each pound costs about hundreds of dollars over the usefullife of the aircraft.

A sealant which can be applied well before assembly and handled whiledry can be made to closer tolerances, without applying excess sealant toassure that there is enough. This potentially avoids the mosttroublesome and costly problems. In addition, the part can be preparedlong before it is needed for assembly, and can be used when it is mostconvenient to the production schedule.

However, attempts to coat one or both of the surfaces, drying thecoating, and then joining them has not previously been successful. Thereasons reside in the stringent conditions the sealant layer mustfulfill.

To be successful for its intended purpose the sealant must be dry sothat it can physically be handled without changing the shape of thesealant layer, or fouling the surroundings. It must not extrude tobecome a nuisance after assembly, and the sealant must ultimately comeinto complete conformity with both surfaces. The surface to which it isdirectly applied will assuredly be fully abutted. However, the exposedother surface of the dry sealant must effectively contact and engage theother surface (or the exposed surface of an opposite sealant layer).Accordingly, in the substantial total thickness required for a practicalsealant, often bridging surfaces from between about 0.005 and 0.01inches apart, down to near contiguity, the dry sealant must bedeformable, but not be liable to substantial cold flow. This is assuredby control of the physical properties of the cured sealant.

In order to be practical, the thickness of the dry layer must beconsistent and readily applied. Surfaces to be covered come in a widerange of sizes and configurations, from long spars and wide panels tointersections with tight corners. While techniques such as spraying,brushing, rolling and flooding can in many situations effectively beused, in general from the point of view of production efficiency,spraying is the preferred method. For this reason sprayable coatings arethe most desirable manner of application.

In order to be practical for production purposes the uncured sealantshould not contain any solvents or volatile materials (although in somecases the use of water as a solvent might be acceptable). If solventsare present they present ventilation problems, as well as fire andexplosion hazards. In addition solvent evaporation from the film canlead to pinholing and film shrinkage with subsequent possibledevelopment of leaks.

The sealant must be strongly adherent to its substrate and be compatiblewith both its substrate, with other sealant compounds which are used toform fillets or beads, and with tackifiers and other adhesives if theyare used. Such other compounds are characteristically applied as abackup as reassurance against leakage through the spacing between thefaying surfaces, and to resist corrosion when a corrosion resistantsubstance is provided as an additive.

While a heat curable sealant is useful in this invention, theapplication of curing temperatures to many substrates could causewarpage or other damage to a substrate such as an aircraft panel. It ispreferable for the sealant to cure at room temperature, for examplebetween about 60 degrees F and about 120 degrees F.

A substantial pot life is desirable when a pre-mixed liquid sealant isapplied. The term “pot life” is less meaningful if the sealant is amultiple component mixture that is mixed in a dispensing nozzle. In thiscase cure time after application is a more appropriate term. While curetimes can vary from minutes to a week or so, a cure time not much longerthan about 16 hours is most practical in a manufacturing venue. Anovernight cure of about 16 hours is about as long as a manufacturingoperation is likely to tolerate. After the cure is completed, thereshould be no limitation on how long the sealant may remain exposed.Certainly it should not be so long as to require a large number of partsto be treated in advance and held in inventory while the sealant cures.

The long list of constraints continues with the requirement that thesealant resist corrosion and solvent attack by many common substances.These substances include such frequently-encountered examples as air,water 1,1,1-trichlorethylene, halogenated hydro carbons, aromaticsolvents such as toluene, common solvents such as ketones (MEK), esters(butyl acetate), alcohols (methyl and ethyl), and hydrocarbon fuels suchas JP8.

Production of a continuous and uniform sealant layer is essential. Forexample, some polymer systems are very sensitive to the presence ofwater, which can generate void inclusions. The sealant must be readilymixed in convenient apparatus in a conventional manufacturingenvironment.

Other requirements, especially for aerospace operations in which the useof this invention will be most frequent is the ability to withstand andoperate over a wide range of temperatures, generally between about −65degrees F and about 250 degrees F or wider. The capacity to be repairedif damaged is essential. Suitability for repair requires that alater-applied application of the sealant can form a continuous bond witha contiguous remaining layer of undamaged sealant.

In view of this array of requirements, which steadily become moredemanding as the complexity, size, and ambient and physical conditionsbecome more severe, it is not surprising that the concept of utilizingdry sealant layers to seal between faying structures has been neglected.The use of more expensive, complicated, and labor intensive sealingtechniques utilizing wet sealants which are later cured in place havebecome the accepted mode despite their cost and other disadvantages.

It is another object of this invention to simplify and reduce the costof a reliable seal between faying surfaces, at the same time providingone which is more reliable and much less likely to require repair.

It is yet another object of this invention to provide structurecomprising a pair of assembled substrates with faying surfaces bridgedby a cured sealant layer according to this invention.

Sealants of this type are often applied to surfaces that are subject tocorrosion by the environments in which they are used. Metal aircraftstructures made of materials such as aluminum, titanium and compositesare examples. To counter this risk, sealants customarily include acorrosion resistant component. By far the most extensively usedsubstances for this purpose are metal chromates, particularly strontiumchromate, zinc chromate, and barium chromate, and their mixtures. Thesefunction well for this purpose, and heretofore have enjoyed widespreadand usually uncritical acceptance.

However, chromates themselves have become environmentally objectionable.Their handling in manufacturing and disposal operations has become moreregulated and troublesome. In some applications the mere presence ofchromates per se has become an issue. Still, the protection of surfacesintended to remain in service for many years, even decades, and in whichthere is very limited access for inspection, and repair is excessivelyexpensive even where it is possible, the use of adequate corrosionresistance components is essential. For this purpose chromates are muchto be preferred, their effectiveness for very extending periods of timehaving been proved long ago.

It is another object of this invention to provide a non-chromatecorrosion resistant element for sealants, especially effective forsealants between faying surfaces, but also effective in other types ofcoatings as well.

BRIEF DESCRIPTION OF THE INVENTION

This invention provides the faying surface of one or of both of a pairof substrates with a layer of solid sealant which is adherent to atleast one of the respective surfaces. The substrates are fastenedtogether by mechanical means so the faying surfaces compress the sealantbetween them to form a fluid seal across the entire spacing between thetwo surfaces, and between the sealant and the faying surfacesthemselves.

According to this invention the sealant consists essentially of anorganic polymer having the following physical properties:

(1) Sprayability in its liquid pre-cured condition, and the ability tocure to a solid at room temperatures.

(2) Resistance to deterioration and solvent attack, and ability to sealagainst passage through it, and between it and the faying surfaces, ofair, water, common solvents and hydrocarbon fuels.

(3) Temperature tolerance to resist thermal decomposition, and toprovide sealing properties at temperatures between about −65 degrees Fand about 250 degrees F.

(4) Compatibility with substrate surfaces of metal such as aluminumalloys, titanium alloys, steels, and structural composite materials.

(5) Adequate deformability to accommodate surface irregularities in thesubstrates which may be exasperated by temperature excursions anddeformation.

(6) Compatibility with sealants customarily used to form beads andfillets.

(7) Ability to join and form a continuous seal with the edges of apreviously deposited and cured layer of a similar or identical sealantmaterial.

(8) Inclusion of a corrosion resistant component which is not achromate.

A sealant for use with this invention comprises an organic polymerpossessing the foregoing physical and chemical properties, in which thephysical properties are derived from a molecular structure whichincludes three-dimensional cross-linkage to form a structural lattice toenhance heat and solvent resistance. Numerous polymeric systems may beemployed, including polyurethanes, polyesters, epoxies, acrylics,synthetic rubbers, and natural rubbers. While all may find utility inmany or most applications, the polyurethanes involve the fewestdifficulties in their preparation and application, which are as well toavoid. Further, they may readily formulated to provide the abovefeatures over a wide range of values. Accordingly, while other systemsfall within the scope of this invention, polyurethanes are much to bepreferred, and will be emphasized in this specification. A corrosionresistant component, a borate, preferably zinc borate, is included.

A structural assembly according to this invention comprises a pair ofsubstrates mechanically joined with their faying surfaces bridged by alayer of said sealant.

The above and other features of this invention will be fully understoodfrom the following detailed description and the accompanying drawings,in which:

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a fragmentary plan view of an assembly of faying surfaces andsealant according to this invention; and

FIG. 2 is a cross-section taken at line 2—2 in FIG. 1.

DETAILED DESCRIPTION OF THE INVENTION

In the drawings a pair of substrates 10, 11 having faying surfaces 12,13, are joined together by a fasteners 14 or other mechanical means. Arivet is shown. Instead threaded pin-collar assemblies could readily beused. A layer 15 of sealant according to this invention is placedbetween them to seal the spacing 16 between them.

The substrates often will be metal sheets or flanges, formed of aluminumalloys, titanium alloys, or steels. The fasteners may be of any suitabletype such as rivets and threaded combinations of pins and collars. Theidentity of the substrates and fasteners are not limitations on theinvention, but are given only as illustrative examples of the uses towhich this invention can be put.

While a layer of sealant can be applied to both surfaces 12 and 13, thiswill usually be an unnecessary duplication of labor. If a layer isapplied to both surfaces, each must be thinner than a layer applied toonly one surface. In that event, each layer will be contiguous to andadherent to its own respective surface. After assembly, the exposedsurface of each layer will conform to the exposed surface of the otherto complete the seal, instead of conforming to the other faying surface.This construction, while not preferred, is within the scope of thisinvention. It is in the nature of this sealant to make a closedcompressive seal. While the previously exposed surfaces of the sealantwill not ordinarily fuse with each other, their mutual resistance toflow and common deformability will assure a fluid-tight seal betweenthem.

When only one layer is used on only one of the faying surfaces, thesealant adheres to its respective surface, and is pressed against theother surface by the assembly procedures. The cured sealant materialresists cold flow, and feels slightly soft, preferably having a Shore Ahardness between about 30 and about 70. This feature aids in maintaininga prevailing sealing force. In modern compressive rivet practice in theaircraft the sealant will usually be compressed between the substrates,and then the rivet will be driven. While the rivet may thereafterelongate a bit after the setting forces are removed, it still willmaintain some compression on the sealant. When a threaded fastener isused, it will maintain a prevailing compression force on the substrates,and thereby on the sealant.

Each of the requisite properties of the sealant, both before and aftercuring, has a substantial range of acceptability. The task of designingthe sealant therefore becomes one of providing it with each of theproperties within the acceptable range. Evidently there can be at leastseveral sealant compositions whose properties will fall within theselected limit ranges. Any of these can be used, but the selection amongthem will often be determined by their convenience in formulation anduse, and in minimized requirements for control of environmentalconditions during mixing and application, and of course minimumtoxicity.

Although other systems are useful and acceptable, generally theurethanes will be much preferred.

Although isocyanates are generally toxic and many monomeric isocyanatesare quite volatile, their product of reaction with polyols is much lesstoxic and readily dealt with. Polyurethanes cured from suitable startingmaterials can be completely acceptable, and are the preferred embodimentof this invention.

As design (selection) criteria, for solvent resistance, resistance tocold flow under the anticipated temperature conditions, and properhardness, a suitable amount of cross-linking is necessary. However, ifthe cross-linking is too great, the cured material will be too hard andtoo brittle for use. If there is insufficient cross-linking there willbe insufficient resistance to solvents or cold flow.

In order to formulate a sealant with the desirable properties asdescribed above it is necessary to strike a proper balance between theamount of cross linking of the final polymer and the chain lengthsbetween cross links in the backbone of the molecule. Too much crosslinkage leads to hard, brittle polymers with insufficient flexibility toperform the required sealing as described above. Too little crosslinking leads to polymers which show poor solvent and chemicalresistance as well as poor resistance to cold flow or creep.

Cross linking is attained in polyurethanes by using multi-functionalmonomeric starting materials (isocyanates or polyols). In this casemulti functional is defined as a functionality greater than two. Inorder to produce a long chain polymer both the isocyanate and polyolmaterials must have a functionality of at least two.

In addition to sufficient cross-linking, the choice of chain length isimportant for the properties of softness and flexibility. Here oneconsiders that if long chain monomers are used to provide softness andflexibility, they must be highly functional. If shorter chain monomersare used, the functionality can be decreased, but there still must besufficient chain length for the cured sealant to have the desiredphysical properties.

There are many possible routes to producing a polymer with the correctbalance or cross linking and chain length. For example long chain diolscan be cross linked with suitable amounts of short chain triols (ortetraols, etc.). Conversely long chain triols can be used to cross linkshort or medium chain length diols.

With the foregoing in mind, the formulator of sealants according to thisinvention will select appropriate chain lengths and functionalities, andmix the reactants prior to application, or mix them as they are beingapplied, perhaps in a spray gun.

Adjustments to the various properties may be made by selecting longer orshorter chains and greater or lesser functionalities.

Additives for various purposes may be including in the pre-cured mix,for example corrosion resistant compounds and catalysts.

Formulations according to this invention do not use chromates forcorrosion resistance. Instead borates, preferably zinc borate, will beused for this purpose. Zinc borate in amounts between about 3% and 30%by weight of the formulation is useful. Its preferred range is betweenabout 6% and about 12% by weight. Percentages less than about 6% areuseful, but at least that amount is to be preferred. Amounts above about12% do not appear to offer enough greater effect to justify their use.About 6%-8% will generally be selected within the preferred range.

Conventional catalysts may be used. Organic metal salts, especiallysalts of tin, and mercury are frequently used. Amines are also usefulcatalysts. Tertiary amines provide for a fast cure that is difficult tocontrol. Secondary amines do not result in a fast a cure and arewell-regarded for the purpose. The most commonly used amine catalystsare primary amines.

Any corrosion resistant additive and any catalyst which is notdeleterious to the composition is within the scope of this invention.The above being merely the preferred examples.

With the foregoing in mind, the following illustrative examples aregiven. The preferred polymer system is a polyurethane.

The various polyols can be obtained as urethane grade materials from avariety of suppliers. The following table shows a few examples of someof the commercially available materials. This table is not intended tobe complete, but only shows a sample of the wide variety of availablematerials.

Functionality Approx. Product Name Supplier (Type) Mol. Wt. Multranol9121 Bayer 2 (diol) 425 Poly G 20-265 Olin 2 (diol) 425 Poly G 20-112Olin 2 (diol) 1000 Multranol 9109 Bayer 2 (diol) 1000 Multranol 3600Bayer 2 (diol) 2004 Poly G 20-56 Olin 2 (diol) 2000 Poly G 20-28 Olin 2(diol) 4000 Multranol 9195 Bayer 2 (diol) 4000 Multranol 9133 Bayer 3(triol) 160 Poly G 70-600 Olin 3 (triol) 282 Poly G 30-280 Olin 3(triol) 615 Multranol 9157 Bayer 3 (triol) 673 Multranol 9144 Bayer 3(triol) 1122 Poly G 30-168 Olin 3 (triol) 1000 Poly G 30-112 Olin 3(triol) 1500 Multranol 9180 Bayer 3 (triol) 1503 Multranol 9187 Bayer 3(triol) 2805 Poly G 30-56 Olin 3 (triol) 3000 Poly G 30-42 Olin 3(triol) 4000 Multranol 9168 Bayer 3 (triol) 3740 Multranol 9181 Bayer 4(tetraol) 291 Multranol 9173 Bayer 5.5 671 Multranol 9185 Bayer 6 3366

The above products are well-known. Their features which are important tothis invention are shown in the foregoing table. “Bayer: refers to BayerCorporation, 100 Bayer Road, Pittsburgh, Pa. 15205-9741. “Olin” refersto Olin Industries.

By combining the proper mixture of high and low molecular weightpolyols, and by using a variety of ratios of diols to polyols withfunctionalities greater than 2, a variety of urethane polymers withdifferent degrees of cross linking and various physical properties canbe obtained. For example backbone chains prepared from high molecularweight diols (to give flexibility) can be cross linked with lowmolecular weight triols to result in urethane polymers with the desiredphysical properties and chemical resistance. Conversely lower molecularweight diols can be cross linked with higher molecular weight triolsand/or tetraols to also obtain a desirable combination of properties.

The above discussion has concerned ways of formulating polyrethanes withthe desired properties by the proper choice of hydroxy compounds. Anequally powerful method of obtaining a variety of properties can be usedby choosing the proper polyisocyanate. However in practice theproperties of the polymer are generally determined by the choice ofhydroxyl compounds, and the isocyanates are chosen for otherconsiderations.

It has been found that in general the strongest but least flexiblepolymers result when aromatic isocyanates are used for theirpreparation. Conversely more flexible but less heat resistant polymersresult when aliphatic isocyanates are used for their preparation.Intermediate properties are obtained when cycloaliphatics are used. Evenmore important than the influence of physical properties by theisocyanates is the resistance to yellowing and weathering when thepolymers are exposed to ultra violet radiation such as occurs in normaloutdoor exposure. Polyurethanes prepared from aromatic isocyanates showpoor resistance to weathering and yellowing whereas those prepared withaliphatic or cycloaliphatic isocyanates exhibit good weatheringcharacteristics.

Another important consideration when choosing the isocyanate used formaking a polyurethane is raw material cost. In general the aromaticisocyanates are the least expensive and the cycloaliphatic ones are themost costly.

As stated above, isocyanates are generally toxic, and the aromaticisocyanates are more toxic than the other types. The primary dangerencountered when working with isocyanates is from inhalation of thevapors. The risk from exposure to vapors can be dramatically reduced ifinstead of using monomeric isocyanates, their volatility is reduced byusing them in a polymeric form. Polyisocyanates are commerciallyavailable as are isocyanate terminated prepolymers. These materials canbe used as substitutes for all or part of the monomeric isocyanates forthe preparation of polyurethanes.

As with the hydroxy compounds, isocyanate materials especiallymanufactured for the preparation of polyurethanes are commerciallyavailable from a variety of manufacturers. A few of the suitablematerials available from Bayer are listed below.

Mondur ML: Aromatic monomeric diisocyanate.

Mondur TDS: Aromatic monomeric diisocyanate.

Desmodur W: Cycloaliphatic monomeric diisocyanate.

Mondur MR: Aromatic polymeric diisocyanate.

Baytec ME-040: Isocyanate terminated polyether prepolymer.

Baytec ME-090: Isocyanate terminated polyether prepolymer.

Baytec MS-041: Isocyanate terminated polyester prepolymer.

Baytec WE-180: Isocyanate terminated aliphatic prepolymer.

In addition the aliphatic hexamethylene diisocyanate can be purchasedcommercially.

The foregoing isocyanates and polyols may be obtained from the BayerCorporation. Further information regarding them will be found in itspublication entitled “Polyurethane Raw Materials ProductIndex—Polyurethane Products”, copyright 1996 which is incorporatedherein in its entirety in this invention for such information, and acopy is being filed along with this application.

The general method for formulating practical polyurethanes is to firstchoose the isocyanate portion of the composition based first on therequirements of resistance to weathering and then on the other factorssuch as cost, toxicity, method of application of the final compositionetc. Once the isocyanate has been chosen, the desirable physicalproperties of the final polymer are obtained by the proper choice of thehydroxy components as described above.

The following are examples of suitable formulation of polyols andisocyanates together with other ingredients, which when mixed will cureto form a useful sealant according to this invention in a suitableperiod of time. Examples 1-6 are urethane systems. Percentages are byweight. The polyols and isocyanates are more completely described in theforegoing lists. DBTDL identifies dibutyl tin dilaurate, which isprovided as a catalyst.

EXAMPLE 1

Poly G 20-56 38.85% Multranol 9109 38.85% Mondur MR 16.3% DBTDL 0.01%Zinc Borate 6%

EXAMPLE 2

Poly G 20-56 70.7% Poly G 70-600 6.7% Mondur FL 18.6% DBTDL 0.01% ZincBorate 4.0%

EXAMPLE 3

Multranol 9109 47.1% Poly G 30-280 19.3% MRS-4 25.6% DBTDL 0.01% ZincBorate 8%

EXAMPLE 4

Multranol 9195 76.8% Multranol 9133 4.1% Mondur MR 16.1% DBTDL 0.015%Zinc Borate 3%

EXAMPLE 5

Poly G 20-56 44.8% Multranol 9185 25.1% MRS-4 12.1% DBTDL 0.01% ZincBorate 18%

EXAMPLE 6

Desmophen 2000 42.5% Multranol 9144 31.7% Desmodur W 19.8% DBTDL 0.02%Zinc Borate 6%

It is well known to those versed in the techniques of successfullyproducing polyurethane films and other products (with the exception offoams) that it is essential to exclude water from the ingredients or theend result will be that soft and weak materials which are full ofbubbles. The reason for this is that isocyanates react with water toproduce carbon dioxide which ends up as bubbles in the final product.For this reason a variety of techniques have been developed to keepwater out of the reactants.

The most common way for water to contaminate the reactants is for it tobe absorbed by them from the surrounding air. Unless the air is dried toan extremely low moisture content (for example a relative humidity of10% or lower) the polyols used for the preparation of the polyurethaneswill absorb sufficient water vapor from the moist air to produce aninferior product. With regard to the isocyanate component of theformulation, the reaction with water referred to above not only producesthe deleterious bubbles, but it also “weakens” the isocyanate so thatthe stoichiometry is thrown off balance, and a soft weak productresults. It is well known that in order to produce strong high molecularweight polyurethane polymers, equimolar amounts of hydroxyl and urethanegroups must be present in the reaction mixture.

Since in the presence of a polymerization catalyst (metal salts, amines,etc.) the hydroxyl and isocyanate materials will react to form apolyurethane, it is essential that these two components be keptseparately until the time that they are purposely allowed to react toform the final desired product.

As was stated above a variety of techniques have been developed to keepwater from the reactants. If the polyols and isocyanates are purchasedcommercially, they are shipped in sealed containers and are blanketed byan atmosphere of dry nitrogen. When these containers are opened for usethe must be opened in a dry atmosphere, or the local surroundingenvironment must be opened in a dry nitrogen (or air) during the pouringor transfer operation. The materials must be transferred into containerswhich have been dried, and which have a dry atmosphere. After thereactants have been transferred to these containers, a dry atmospheremust be maintained by flooding with dry gas before closing and sealing.The opened containers from which the reactants have been poured mustalso be flooded with dry gas before resealing. One technique which hasoften been used successfully is to transfer the materials from onecontainer to another by the use of vacuum. A slight vacuum is drawn inthe receiving container, and the material is transferred from thestorage container to the receiver by “blowing” it from the one vessel tothe other by taking advantage of the pressure differential.

By the use of the various techniques just described, the products givenin the above examples are prepared as follows: The polyol component (Acomponent) is prepared by mixing together the polyols given in eachexample along with the polymerization catalyst (DBTDL in the examplesgiven) and the chromate corrosion inhibitors. If the material is to besprayed to form a thin film, the chromates must be milled into a polyolcomponent using a suitable mill such as a ball mill, sand mill or threeroll mill to a paste in which the pigment is finely dispersed. Generallymilling the paste to a Hegmen gauge reading of 6 or higher results in asatisfactory dispersion. Care must be taken during the milling processto ensure that no water is allowed to contaminate the paste.

The isocyanate (B component) is weighed in the proper amount, and the Aand B components are mixed together just prior to application. The mixedproduct is allowed to polymerize to cure to the final product. In theexamples given an overnight cure will result in a satisfactory material.The cure can be slowed up or hastened by adjusting the amount or type ofcatalyst. For example the cure can be slowed down by decreasing theamount of tin catalyst used, or by substituting an amine catalyst forthe tin catalyst. The cure can be accelerated by increasing the amountsof tin catalyst, using a different organo tin salt (for example dibutyltin dichloride) or by using an amine catalyst along with the DBTDL.

The final mixed product is applied to the substrate by a suitabletechnique such as brushing, spraying, drawing down a film, trowellingetc. As stated above spraying is usually the preferred method ofapplication.

Although the preferred polymers to use for this invention arepolyrethanes, any polymer which can be formulated to give a softflexible material with the correct physical properties to show adequatesolvent resistance, temperature resistance, etc., as described above canbe used. Satisfactory useful polymer types in addition to thepolyurethanes include polyesters, epoxies, acrylics, silicones, naturaland synthetic rubbers, polybutadienes and certain vinyl materials.

This invention is not to be limited by the embodiments shown, in thedrawings and described in the description, which are given by way ofexample and not of limitation, but only in accordance with the scope ofthe appended claims.

I claim:
 1. A sealant for forming a fluid seal between the fayingsurface of two metallic substrates that are compressively andmechanically joined together, said sealant being liquid in its pre-curedcondition and a dry solid in its cured condition, the transition frompre-cured to cured condition occurring at room temperature with a curetime not less than about 10 minutes and not more than about 16 hours,said sealant when cured having a Shore A hardness between about 30 andabout 70, being impermeable and resistant to chemical attack by air,water, common solvents and petroleum fuels, resistant to compressivecold flow but deformable to conform to an abutting one of said surfacesor with a layer of similar sealant on the other of said surfaces, andflexible and resistant to a temperature range between about −65 degreesF and about 250 degrees F, said sealant comprising a cured copolymer ofan isocyanate and a polyol, both the isocyanate and polyol having atleast two functionalities, and the isocyanate and polyol when mixedbefore curing containing a dibutyl tin dilaurate catalyst in an amountrespective to an intended cure rate, said sealant including a metalborate corrosion resistance component.
 2. A sealant according to claim 1in which the isocyanate and polyol when mixed before curing containssaid metal borate.
 3. A sealant according to claim 2 in which the metalborate is zinc borate in amounts by weight between about 3% and about30% of the sealant.
 4. A sealant according to claim 3 in which the zincborate is in amounts between about 6% and about 12% by weight ofsealant.
 5. A structural assembly comprising a pair of metallicsubstrates each having a faying surface, said faying surfaces facingtoward one another; a layer of sealant between said faying surfaces andadherent to at least one of them; and fasteners compressively holdingsaid surfaces against said sealant layer to form a seal between saidsurfaces; said sealant comprising a sealant for forming said sealbetween said faying surfaces said sealant being liquid in its pre-curedcondition and a dry solid in its cured condition, the transition frompre-cured to cured condition occurring at room temperature with a curetime not less than about 10 minutes and not more than about 16 hours,said sealant when cured having a Shore A hardness between about 30 andabout 70, being impermeable and resistant to chemical attack by air,water, common solvents and petroleum fuels, resistant to compressivecold flow but deformable to conform to an abutting one of said surfacesor with a layer of similar sealant on the other of said surfaces,flexible and resistant to a temperature range between about −65 degreesF and about 250 degrees F, said sealant comprising a cured copolymer ofan isocyanate and a polyol, wherein both the isocyanate and polyol haveat least two functionalities, and the isocyanate and polyol when mixedbefore curing contain a dibutyl tin dilaurate catalyst in an amountrespective to an intended cure rate, said sealant including a metalborate corrosion resistance component.
 6. A structural assemblyaccording to claim 5 in which the isocyanate and polyol when mixedbefore curing contains said metal borate.
 7. A structural assemblyaccording to claim 6 in which the metal is zinc borate in amounts byweight between about 3% and about 30% of the sealant.
 8. A structuralassembly according to claim 6 in which the zinc borate is in amountsbetween about 6% and about 12% of the sealant.